5th International DAAAM Baltic Conference
"INDUSTRIAL ENGINEERING – ADDING INNOVATION CAPACITY OF LABOUR
FORCE AND ENTREPRENEURS"
20–22 April 2006, Tallinn, Estonia
FATIGUE OF DUPLEX STEELS IN CORROSIVE ENVIRONMENT
Saarna, M.
Abstract: Duplex stainless steels (DSS)
are prime candidates for paper and pulp,
off shore and mining industry due to their
high corrosion fatigue resistance
accompanied by good mechanical
properties. The aim of the study is to
obtain the fatigue data for two powder
metallurgical (P/M) DSS steels in
modified TIP 0402-09 type II corrosive
environment resembling white water
found in pulp and paper industry. The
specimens were manufactured from two
types of commercial DSS one hot rolled
type and one P/M hot isostatically
pressed (HIP-ed) type. Axial fatigue
testing method at 15 Hz on a servohydraulic testing machine was utilized for
determining the fatigue limit and S/N
curve. Fracture surfaces were examined
using scanning electron microscope and
light optical microscope.
Key words: Duplex steel, corrosion,
fatigue, fracture.
1. INTRODUCTION
In pulp and paper industry duplex
stainless steels (DSS) are prime
candidates for use in paper machine
suction rolls 1]. Suction rolls are used in
“ white water” removal and to control the
wet paper web during the paper making
process 2]. “White water” is corrosive
agent
containing Chloride
(Cl-1),
2Thiosulphate (S2O3 ) and Sulphate
(SO42-) ions 6].
Fatigue is progressive, localized,
permanent structural change that occurs
in materials when subjected to fluctuating
stresses and strains that may result in
development of cracks or fracture after
sufficient number of cycles of cracks or
fracture after sufficient number of cycles of
fluctuations [3]. Fatigue testing can be
carried out in several control regimes - stress
control and strain control. Former is used in
case of high cycle fatigue, latter low cycle
fatigue. There are several loading types
possible: axial, torsion, rotating [4].
The aim of the study is to obtain the
fatigue data for two commercial DSS steels
in modified TIP 0402-09 type II corrosive
environment and to give a reference to
literature results obtained in TIP 0402-09
type I corrosive environment.
2. DUPLEX STEELS
Tested Duplok22 and 3RE60 SRG are low
alloy DSS-s and were supplied by Metso
Powdermet OY.
They contain approximately equal separate
volume fraction of ferrite (α) and austenite
(γ), which grant them unique corrosion
resistance along with good mechanical
properties [3]. In modern DSS the α and γ
ratio can be high as 40/60 respectively [7].
DSS are manufactured as forged, cast,
wrought and P/M products. The initial
powder size before hot isostatic pressing
(HIP-ing) is <250 μm in the case of powder
metallurgy (P/M) HIP-ed Duplok22 [3]. Fine
and
homogeneous
microstructure
is
preserved during the HIP-process (Fig.1a).
The hot rolled 3RE60 SRG microstructure
shows that the ferrite and austenite
distribution is not as fine as in Duplok22
(Fig. 1b).
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When testing DSS in corrosive environment
the fatigue crack could initiate at the pitting
marks, which act as a stress concentrators.
Pitting marks occur when localized corrosion takes place.
Resistance to pitting corrosion can be
evaluated by the pitting corrosion resistance
number (PREN), which is calculated in
literature [5] according to Eq.(1)
PREN = wt% Cr + 3.3 (wt% Mo) +
+ 16 (wt% N)
(1)
(a)
DSS-s are divided into two groups according
to PREN. Duplex PREN <35 and super
duplex PREN >40.
Both 3RE60 SRG and Duplok22 belong to
the first group with PREN number about 34
and about 29 respectively [3,5].
Table 1. Chemical composition of the tested
DSS-s [2,3].
Steel
C, % Mn,% Cr,% Ni,% Mo,% N,%
grade
2.2 Corrosion resistance
DSS have excellent resistance to stress
corrosion cracking compared to conventional austenitic stainless steels [3].
This can be explained by the alloying
elements especially Cr, Mo and N and
unique behaviour and portioning of the
alloying elements of the two phases [3].
Duplok22
2.1 Chemical composition
DSS-s contain Ni to stabilize the austenite and to form a duplex structure, Cr to
give them corrosion resistance and
stabilise ferrite, Mo to enhance corrosion
resistance and stabilize ferrite, N and Mn
to stabilize austenite [3]. N is almost
completely dissolved in the austenite [3,5].
Duplok22 has higher alloying element
concentration compared to 3RE60 SRG.
The tested material chemical composition
is shown in Table 1.
3RE60
SRG
(b)
Fig. 1 Microstructure of tested steels
0.02 1.50 18.50 4.90 2.8 0.08
0.03 0.60 22.0 6.10 3.05 0.15
2.3 Mechanical properties
Low-nitrogen DSS good mechanical strength
originates from the ferrite phase and high
impact toughness from the austenite phase
compared to conventional austenitic stainless
steels [3,5]. The mechanical properties are not
affected by the thickness or the orientation
of the microstructure in P/M HIP-ed DSS-s
[3]. Austenite is reported to be the weaker
phase even when N content is as high as
0.32%.
The mechanical properties of the tested
DSS-s are shown in Table 2.
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Table 2. Mechanical properties of tested
DSS-s [2,3].
Steel grade
3RE60 SRG
Duplok22
Tensile
strength,
MPa
719
739
Yield
strength,
MPa
440
499
3. EXPERIMENTAL
3.1 Test method and equipment
To determine the fatigue limit of DSS-s
axial push-pull method with constant
stress ratio R= –1 was used. The testing
was carried out under load-controlled
regime at 15 Hz frequency. Instron 8516
type 100 kN servo-hydraulic test machine
was used.
Test chamber was made of stainless steel
(Fig. 2) and coated with Teflon. Test
chamber window is made from acrylic
plastic. A membrane pump was used to
circulate the test solution about twice an
hour (test chamber volume about 0.7 l).
Specimens were submerged by the
parallel part into the test solution.
Test end criterion was failing of the
specimen at any given stress level or
reaching 107 cycles.
Fig. 2 Test cell
The variable was the maximum stress
level Smax, and output cycles count. The
Smax values were chosen to cover the S/N
curve from 104 to run out (107) cycles.
The specimens were round Ø 10 mm at the
grips, Ø 6 mm at the 24 mm long parallel
part and 150 mm in total length.
The specimens were mechanically polished
at the parallel part to remove scratches using
a polishing wheel and abrasive. Diameter
and surface roughness Ra was measured.
The required Ra was <0.6.
3.2 Testing environment
Testing environment was a modified TIP
0402-09 type II test solution. Solution
containing NaCl, Al2(SO4)3·18H2O and
Na2S2O3·5H2O dissolved in distilled water.
The test solution pH was adjusted to 3.50 by
adding 1vol% H2SO4.
The test solution composition was following
(prior to pH adjustment):
Cl-1 – (1000 ppm);
SO42- – (800 ppm);
S2O32- – (200 ppm).
The expiration time for a solution was three
days.
The test solution pH tended to increase about
0.1 pH units per 24 hours and then stabilise
at about pH 3.70.
4. TEST RESULTS
According to the material specification [2]
the fatigue limit for 3RE60 SRG is 200 MPa
at 5Hz and 265 MPa at 25 Hz, obtained with
test solution containing 400 ppm Cl-1, 250
ppm SO42- ions and pH 3.50.
Fatigue limit, as it follows from Fig. 3, for
the 3RE60 SRG was 250 MPa is mean value
when compared to the literature results when
taking account the testing frequency.
Fatigue limit, as it follows from Fig. 4, for
the tested Duplok22 was 350 MPa.
According to the material specification [2]
the fatigue limit for Duplok22 is 275 MPa
(109 cycles).
This result is obtained with less corrosive
test solution containing 100 ppm Cl-1, 1000
ppm SO42- ions and pH 3.5 (TIP 0402-09
type I).
Duplok22 higher fatigue limit compared to
3RE60 SRG is due to its fine microstructure,
which in inhibits the growth of a fatigue
291
crack and also due its higher alloying
element content [3]. Fatigue cracks tend
to grow in ferrite, austenite phase tends to
retard the crack [3].
355
345
Smax
335
Fracture surface analysis revealed that
fatigue crack initiation could be traced to
one certain initiation point in all the tested
specimens (Fig. 5, 6).
In the case of low cycles counts, 104 and 105
cycles, the initiation site was an inclusion
situated on or near the surface (Fig. 5, 6).
325
315
305
295
-
2
4
6
8
10
Number of cycles
Million of
cycles
Fig. 3 3RE60 SRG S/N curve
460
440
Smax
420
400
a) 3RE60 SRG 5·105 cycles (LOM)
380
360
340
-
2
4
6
8
Number of cycles
10
Million of
cycles
Fig. 4 Duplok22 S/N curve
5. FRACTURE SURFACE
ANALYSIS
In the case of high-cycle fatigue the
initiation of cracks takes mainly place at
non-metallic inclusions situated at or near
the surface of a specimen [3]. In corrosive
environment pitting marks may enhance
the initiation of cracks [3].
The influence of inclusions is more
important in the case of P/M HIP-ed
DSS-s like Duplok22 [3]. HIP-ing process
is used to obtain fully dense materials [8].
Certain period of time is needed for
corrosion to occur. Under high frequency
testing the crack tip may not be exposed
to corrosive media for significant time
[9]. In the tested materials first signs of
pitting marks occurred within 24 hours of
being exposed. Fracture surfaces were
examined using scanning electron
microscope (SEM) and light optical
microscope (LOM).
b) 3RE60 SRG 5·105 cycles (SEM).
Fig. 5 Crack initiation point
Fig. 6 Crack initiation point 3RE60 SRG
6·104 cycles (SEM)
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In case of 106 cycles the effect of
corrosion is more pronounced. A pitting
mark may act as a crack initiation point
(Fig. 7).
Fig. 7 Pitting mark as crack initiation
point 3RE60 SRG 1.5·106 cycles (SEM)
6. CONCLUSION
In the case of corrosion fatigue testing of
the duplex stainless steels the test
parameters have significant influence on
the test results especially the testing
frequency.
Corrosion needs time to form pitting
marks that could act as fatigue crack
initiation spots.
Both tested steels (3RE60 SRG and
Duplok22) were affected by the pitting
corrosion when exposed to the test
solution for at least 24 hours.
Duplok22 higher fatigue limit is due to its
fine microstructure in both TIP 0402-09
type I and type II solution.
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4. Mechanical Testing and Evaluation. ASM
handbook, 8, 2000
5. El-Yazgi, A. A., Hardie, D, Stress
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